Phantos are large, seemingly-living masks that can often be found guarding various treasures, most often keys. Its name appears to be a shortened form of the word "phantom", another word for ghost or spectre. Originally, Phanto sported a very solemn demeanor in Yume Kōjō: Doki Doki Panic, while starting with Super Mario Bros. 2, Phanto was given an evil grin.

Phantos appear as semi-common enemies in Super Mario Bros. 2 (and its subsequent remakes and ports), often in caves or in vases. Phantos simply remain immobile unless the key they are guarding moves. Once this happens, they ceaselessly chase whoever stole its key. Phantos cannot be avoided and can only be escaped by using the key it was guarding to unlock a door. If the player simply drops the key while a Phanto is chasing them, it flies away and leave them alone, at least until the key is picked up again. Phanto can follow the player through different screens and rooms, even into Subspace. Phantos can only be defeated with a combination of a Stop Watch and a Super Star, as the Super Star alone does not affect it. After destroying a Phanto, however, a new one replaces it if the player enters another room while still carrying the key. A number of Phantos are also shown in various rooms, like the keys' rooms and Wart's room, though they do not do anything.

Despite the sprite for Phanto being modified between Yume Kōjō: Doki Doki Panic and Super Mario Bros. 2, artwork for him usually retains his unsmiling appearance from the Japanese title. The original NES version of Super Mario Bros. 2 featured red and white Phantos, but subsequent remakes have also featured golden Phantos. In the SNES and GBA versions of Super Mario Bros. 2, larger Phantos can be seen in the background of certain vase interiors. The eyes of these larger Phantos will glow red repeatedly once the key is grabbed. In Super Mario Advance, if the player drops the key or loses a life, the Phanto appears enlarged as it flies away, creating the illusion of it flying towards the camera.